This would be what you paid with the trade; for example, the new vehicle cost $10,000 and you got $3,000 for your trade-in, you paid $7,000 cash with the trade-in.
If you traded in one Business Vehicle for another, Edit your old vehicle, indicating you 'stopped using it for business' and the date. Answer the questions about trade-in amount received and any expenses for 2018.
If you have been claiming expenses for the first vehicle, you may have a loss/gain, depending on what value (cost basis) was left on the vehicle and what you received as a trade-in.
Leave the page asking about 'gain/loss' blank; TurboTax will calculate this for you.
Add the new vehicle, with the Cost of that vehicle and when you started using it for business.
See attached screenshots for help.